1. Field of Invention
This invention relates to a method of ameliorating the residual stresses in metallic duplex tubes and the like (hereinafter, "pipe" will be used for the generic terms "tubing" and "tube," and the term "double pipe" will be used for "duplex tube") and an apparatus therefor, each of the metallic double pipes being made of either the same austenitic stainless steel or different one (in either case, referred to as "austenitic stainless steel" hereinafter). In particular, this invention is concerned with a method of ameliorating the residual stress occurring in the vicinity of the weldment that joins the internal surface of the outer pipe, or the main pipe, and the inner pipe, or the thermal sleeve inserted into the outer pipe, as well as the apparatus therefor.
2. Description of the Prior Art
It is known that in austenitic metallic materials, such as austenitic stainless steel or the like that are often used in nuclear plants and chemical plants, stress corrosion cracking (referred to as "SCC" hereinafter) takes place rapidly under the simultaneous presence of tensile stress and a corrosive environment. It is also known that, since SCC takes place only when three factors, namely, the austenitic structure, the tensile stress, and the corrosive environment, are simultaneously present, SCC can be prevented by ameliorating any of these three. Here, it is to be noted that, even though the austenitic structure and the corrosive environment are subject to service requirements and are thus difficult to change, the tensile stress may be ameliorated by modifying the processing method.
In the conventional method of ameliorating the residual stress in a seamed pipe of austenitic stainless steel, the outer surface of the pipe is induction-heated while the inner surface is cooled by water so as to give rise to a temperature difference across the pipe wall that will produce a thermal stress exceeding the yield strength of the steel. This provides a way of ameliorating the residual stress by producing a residual compressive stress at the inner side of the seamed portion of the pipe and thereabout, where the residual tensile stress is apt to occur.
Although the above-mentioned treatment method is suited for austenitic stainless steel pipes of simple forms, such as straight pipes, it has been found unsuitable for use with a complex form such as a double pipe, consisting of a main pipe as the outer pipe and a thermal sleeve as the inner pipe, the root of the latter being welded to the inside of the former. This is because, since the thickness of the part where the main pipe and the thermal sleeve are welded together becomes far greater than that of the main pipe itself, the temperature distribution in the main pipe, produced by the induction heating mentioned above, becomes uneven, and the magnitude and the direction of stress generated by such temperature difference deviate from what was intended.
Also, there is a danger that the neighboring parts of the thermal sleeve root weldment may become partially sensitized by the welding heat, making them worse rather than better by the treatment.